In existing aircraft, bypass engines such as turbojet engines are suspended beneath the wing by complex attachment devices also referred to as EMS (Engine Mounting Structures) or attachment pylons. The attachment devices usually employed have a primary structure, also referred to as a rigid structure, often produced in the form of a single box, namely one made up of the assembly of upper and lower spars joined together by a plurality of transverse ribs situated inside the box. The spars are arranged on the upper and lower faces, while side panels close the box on the lateral faces. In addition, the attachment pylon is arranged in the upper part of the engine, between the latter and the wing box. This position is referred to as the “12 o'clock” position.
As is known, the primary structure of these pylons is designed to allow the static and dynamic loads generated by the engines, such as the weight, the thrust or even the various dynamic loads, notably those associated with cases of failure such as: loss of blades (FBO), collapse of the front landing gear, hard landing, etc. to be transmitted to the wing.
In known attachment pylons of the prior art, the transmission of loads between its primary structure, known in the form of a single box, and the wing, is performed in the conventional way by a set of mounts comprising a front mount, a rear mount, and an intermediate mount notably intended to react thrust loads generated by the engine.
In order to achieve this, the intermediate mount is intended to react thrust loads and is also referred to as a “spigot” mount and is generally embodied by a ball fixed in the rear upper spar of the rigid-structure box between the front mount and the rear mount. This spigot mount also comprises a shear pin or peg fixed under the wing of the aircraft by an insetting fitting, so as to be able to be housed in the ball. The insetting fitting is generally fixed to a lower part of the wing box, usually the lower spar of the wing box.
In recent bypass engines, the high bypass ratio desired has led to an extremely high bulkiness because an increase in the bypass ratio unavoidably leads to an increase in the diameter of the engine and, more particularly, to an increase in the diameter of the fan casing thereof.
Thus, with a ground clearance which is fixed so that it remains acceptable from a safety standpoint, the space left available between the wing element and the engine becomes increasingly small, or even non-existent in the case of engines with a high bypass ratio. As a result, it may prove difficult to install the attachment pylon and the various wing mounts in this remaining vertical space usually devoted to such installation.
The way in which bypass engines have evolved has therefore had the detrimental effect of imposing a reduction on the vertical dimensions of the attachment pylon, notably so as to be able to maintain sufficient space for installing the front and rear mount fittings and the intermediate mount inset fitting. The large dimensions of this intermediate fitting are necessitated by the need to react the engine thrust loads, namely loads oriented in the longitudinal direction of this engine, and those oriented in the transverse direction thereof. By way of indication, it is recalled that the longitudinal direction of the engine corresponds to the direction of the main axis of rotation of the propulsion system.
However, the options for reducing the vertical dimensions of the attachment pylon are limited. Specifically, the rigid structure of this pylon, also referred to as primary structure, needs to have sufficient dimensions that it is able to afford mechanical strength capable of withstanding the transmission of load from the engine toward the wing element, with small deformation under stress with a view to not impairing the aerodynamic performance of the propulsion system.
In the prior art, multiple solutions have been proposed for bringing the engine as close as possible to the wing element from which it is suspended, this being with a view to maintaining the ground clearance required, notably with regard to the risks of ingestion and collision, also known as the FOD (Foreign Object Damage) risk. Nevertheless, these solutions need constantly to be improved upon in order to adapt to suit the increasingly high diameters of fan casing adopted in order to meet bypass ratio requirements.
By way of indicative example, document FR 2 993 535 discloses an attachment pylon, the primary structure of which is produced from two diametrically opposed side beams arranged one on each side of a vertical mid-plane of the engine. The primary structure is supplemented by a connecting structure which directly joins the two beams together, these incidentally each being fixed at their front ends to the engine casing and at their rear ends to the wing box. The intermediate structure adopts the form of several bows connecting the two beams, passing along an imaginary surface of circular cross section corresponding substantially to the surface externally delineating the bypass flow path, also referred to as the “internal nacelle”. These bows therefore extend over angular sectors of the order of 180°.
The arrangement proposed in this document FR 2 993 535 notably makes it possible to limit aerodynamic disturbances within the bypass flow path. Furthermore, by positioning the beams laterally it is possible to bring the engine as close as possible to the wing element, particularly by comparison with the conventional solutions in which the box-shaped primary structure is arranged in the 12 o'clock position.
Nevertheless, such a primary structure has a high overall bulkiness particularly because of the presence of the bows that connect the side beams. This bulkiness may make installing surrounding elements such as the nacelle, the engine auxiliary systems, the thrust reversers, the mobile leading edge flaps, etc. more difficult. As a result, there remains a need to optimize the bulkiness of such primary structures of the attachment pylon.